Half a billion people in northern China will likely live an average of 5.5 years less than their southern counterparts because of heavy air pollution caused by coal burning, according to a new study. Researchers from China, Israel and the United States were able to compare health effects between people in the north and south parts of China because the country had a decades-long policy of providing free coal only in the colder north.

Harvard School of Public Health biostatistics professor Francesca Dominici, quoted in a July 9, 2013 Associated Press article, called the study “fascinating” because it allowed researchers to approximate a randomized experiment in a situation “where a randomized experiment is not possible.”

Dominici was not involved in the study but has researched the health effects of fine particulate matter, or PM2.5—particles with diameters of 2.5 micrometers or less—in the U.S. She said she wasn’t surprised by the new findings, given China’s high pollution levels. “In the U.S. I think it’s pretty much been accepted that even small changes in PM2.5—much, much, much smaller than what they are observing in China—are affecting life expectancy,” she said.